Other Expenses - Other Expenses Breakout (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Component of Operating Other Cost and Expense [Line Items]
Other Cost and Expense, Operating	$ 23.2	$ 21.6	$ 21.4
Clearance Fees	1.6	1.7	1.8
Contingent Consideration [Member]
Component of Operating Other Cost and Expense [Line Items]
Other Cost and Expense, Operating	3.0	2.9	4.7
Insurance Expense [Member]
Component of Operating Other Cost and Expense [Line Items]
Other Cost and Expense, Operating	1.8	1.7	1.5
Advertising, Meetings and Conferences [Member]
Component of Operating Other Cost and Expense [Line Items]
Other Cost and Expense, Operating	2.3	2.5	1.8
Non-trading hardware and software maintenance and software licensing [Member]
Component of Operating Other Cost and Expense [Line Items]
Other Cost and Expense, Operating	2.7	2.2	2.8
Office supplies and printing [Member]
Component of Operating Other Cost and Expense [Line Items]
Other Cost and Expense, Operating	1.2	1.3	1.1
Taxes Other than Income and Excise Taxes [Member]
Component of Operating Other Cost and Expense [Line Items]
Other Cost and Expense, Operating	3.8	4.0	2.8
Other Operating Expense [Member]
Component of Operating Other Cost and Expense [Line Items]
Other Cost and Expense, Operating	$ 6.8	$ 5.3	$ 4.9